Anodizing process and system



Jan. 5, 1960 M. A. MILLER 2,920,018

ANoDIzING PRocEss AND SYSTEM Filed April 22, 1957 79 Power .gaf/ jg 117%5 'I I a4 90? United States Patent O ANoDIziNG PROCESS AND SYSTEM Myron A. Miller, North Hollywood, Calif., assgnor to Electro-Chem Manufacturing Co., Inc., Los Angeles, Calif.

Application April 22, 1957, Serial No. 654,330

5 Claims. (Cl. 204-56) The present invention relates to an improved anodizing process and apparatus for producing inorganic oxide coatings on metallic bodies, such as aluminum and other materials capable of being anodized.

In recent years, certain materials capable of acquiring anodic coatings have come into widespread use. Such coatings are formed on the particular materials by an electro-chemical modilication of its surface to become an integral part of the material itself. The resulting coating is highly resistant to corrosion and it is nonreactive. y

The electro-chemical modification of the material surface referred to above is achieved by means of a process known as anodizing. In the anodizing process, the workpiece is emmersed in a bath of a suitable electrolyte, and it forms the anode of an electric circuit through which a current is passed. Anodizing has been applied most successfully to pure aluminum and certain aluminum alloys, and the process and apparatus of the present invention will be 'described in conjunction with these materials. The technique, however, has also been applied to zinc and magnesium, and to other materials. It will be evident as the description proceeds that the apparatus and process of the invention are applicable to any material capable of acquiring an anodized coating.

The tendency of an aluminum body to form an oxide film or coating when the body is made the anode of an electrolytic cell, has been known for many years. The fact that this oxide coating exhibits uni-directional current characteristics has also been known for a long time. However, it was only within the last 20 years that processes have been devised for producing relatively thick and commercially usable anodic coatings for corrosion protection. Also, it is only within the last few years that anodizing processes have been brought within the realm of commercial practibility.

The original anodizing process, which was developed over 20 years ago, utilizes a chromic acid bath as the electrolyte. This original process is still in use today in some countries, notably England. In the United States, however, the original process has been superseded to a large extent by processes using suphuric or oxalic acid baths, or a combination of the two acids.

The characteristics of the anodi-c coating are largely determined by the particular electrolyte that is used. For example, when an electrolyte solution of boric acid is used, the resulting anodic oxide coatings are substantially indissoluble in the electrolyte. This electrolytic solution produces thin, impervious oxide coatings as are required in electrolytic condensers and the like. However, these particular coatings are not particularly corrosion or abrasion resistant, v

On the other hand, when the electrolyte is sulphuric, oxalic, or chromic acids, the resulting oxide coating is continually being dissolved by the electrolyte as the process is being carried out. This results in the production of a relatively thick, but porous, coating.

The anodic coatings which result vfromthe use of sul- Patented Jan. 5, 1960 ICC phuric acid, or the other acids described in the immediately preceding paragraph, afford high protection from corrosion and they are highly resistant to abrasion. However, as noted, these coatings are formed in electrolytes which exert an appreciable dissolving action on the coating as it is being formed. This counteracting solvent action produces problems which will be discussed subsequently. Moreover, the coatings are often relatively soft and unduly porous when formed by most of the prior art processes. The desired end in this area is to produce an anodic coating that is thick, hard and not unduly porous.

One known prior artanodizing process uses a 3% solution of chromic acid as the electrolyte bath. The bath is maintained at 40-45 C. by means of refrigera tion cooling coils while the process is being carried out. Such cooling coils are required because of the liberation of heat by the process 'and the need to prevent undue heating ofthe electrolytewith corresponding increased dissolution of the anodic ilm by the electrolyte. At the beginning ofV the operation, the voltage is held at around 40 volts, and this voltage is maintained for the first 15 minutes. The voltage is then raised to 50 volts so as to maintain a constant current flow through'the system as the oxide anodic layer builds up and increase the anode resistance. The total time required in this prior art process is of the order of one hour, and the current density ranges from 2.5-4 amps/sq. ft. Films of the order .1-.2 thousandths of an inch can be formed by this prior art process The limitation on the thickness of the anodic layer or film resides in the fact that a particular current llow must be maintained in order that the loyer may be built up faster than it is being dissolved by the electrolyte. However, as indicated above, the voltage must be continually increased to maintain this constant current as the essentially non-conductive surface layer is built up on the anode surface. This increased Voltage increases the temperature at the anode surface which increases the reaction ability of the electrolyte with the layer. V'The limit of thickness of the layer is reached when the current fiow and resulting temperature at theanode reaches a limit such that an increase in voltage to maintain a particular -current ow produces a greater dissolving effect inthe electrolyte, so that such voltage increase results in no increase in layer thickness.

A more recent process has been developed. This later process uses sulphuric acid as an electrolyte, and it is capable of producing hard transparent coatings ou aluminum which are materially thicker than those produced by the process described above.

After the anodizing operation, the resulting oxide coating is' usually treated by boiling water which transforms ,theY anodic aluminum oxide tilrn (A1203) into aluminum anhydrate. The result is a reduction in film porosity and an improvement in its resistance to corro'- sion. This latter operation must be carefully controlled to obtain a uniform product.

In general, and as mentioned briefly above, in the production of anodicl coatings the electric current through the electrolyte tends to form the anodic film, whereas the electrolyte itself tends to counteract the action of the current and dissolve the film. -An increase in the tem- 'at a relatively low temperature so as to reduce the tendency of theY electrolyte to dissolve the oxide coating. g The electric current passed through the electrolyte must penetrate the anodized oxide film to the conductive: anode body'.V This is because anodizing takes place between the base surface of the anode and the anodized oxide film.

This is different from electrodepositing in which a conductive layer is built up on the base electrode and the electrodepositing action takes piace on the surface of the deposited layer. Therefore, problems are encountered in anodizing processes that are not met in electrodepositing.

For example, the anodic film is a good dielectric and, therefore, as the lm is built up by the anodizing process, the electric resistance of the anode increases. This why the voltage, as indicated previously, has to be increased as the operation progresses so as to maintain the constant current flow which is essential to build up the desired lm. However, as noted, the increase of voltage results in an increase in the amount of heat at the anode surface. This in turn increases the rate at which the coating is dissolved by the electrolyte. A limiting lm increase is reached, therefore, when the rate of chemical dissolution of the film by the electrolyte is equal to the rate of film growth.

Therefore, to produce a thick, hard anodized lm it is necessary to increase current density in some manner without an accompanying excessive increase in the ternperature of the anode surface.

There also is a problem of initial passivity of the anode, due to polarization. This latter problem is of more significance when it is attempted to anodize highly alloyed aluminum materials. The phenomenon of anode polarization is Well known to the art. Anode polarization produces a tendency for the workpiece to burn and be destroyed or severely damaged during the anodizing process. Moreover, the ability of any anodizing process to form a thick and hard anodic lilm is materially reduced by anode polarization.

Some of the problems outlined in the preceding paragraphs have been solved to some extent by superimposing an alternating current on the direct anodizing current which is passed through the anode formed by the Workpiece and through the electrolyte. The superimposed alternating current is in the form of a ripple on the direct anodizing current, and it does serve to overcome the adverse effects of anode polarization to some extent. Also, the use of the superimposed alternating current does permit higher current densities to be used for Yan increase in speed in the work cycle which results in harder and thicker coatings than those produced by the previous methods.

The use of the superimposed alternating current, as noted above, results in the production of a harder and thicker anodic coating than those previously attainable. However, the provision of the superimposed alternating current on the direct anodizing current requires relatively complicated, expensive and inexible circuitry. Also, this superimposing of the alternating current does not solve the problem of heating at the anode surface which occurs, as noted above, when the current density is increased. As pointed out, this heating of the anode surface is a limitation on the thickness and hardness to which an anodizing film can be built up on the workpiece.

In the process of the present invention, the actual current flow through the anodizing apparatus is pulsed. This pulsing of the entire anodizing current not only overcomes the anode polarization problems described above, but it also provides a cooling period between each interval of current ow.

The anode cooling effect provided by the pulsing action of the present inventionris not attainable by refrigeration cooling coils in the electrolyte. There is a limit to the extent to which the cooling coils can reduce the temperatureof the electrolyte and yet permit the electrolyte to carry out its electro-chemical function in the anodizing process. However, even though the temperature of the electrolyte has been reduced to its absolute minimum limit for satisfactory electro-chemical action by the cooling coils, there is still excessive heat developed at the anode surface. As described above, this heat increases the attack of the electrolyte` on the anode which renders the anodic lm soft and porous, and which also sets a limit to the thickness with which the anodic film can be built up.

The pulsing of the anode current in accordance with the present invention provides a direct cooling at the anode surface. This permits higher current densities to be used without concomitantly excessively increasing the reaction of the attack of the electrolyte on the anodic film. Therefore, the direct result is the production of thicker and harder anodic lms than those produced by the prior art processes. The invention provides, therefore, an improved and simplified anodizing process which results in the production of relatively thick and hard anodized films, and in which such production is carried out in a quicker work cycle than that required bythe prior art processes.

The invention also provides improved apparatus for carrying out the pulsating anodizing action of the inventive process. The apparatus is relatively simple and inexpensive to construct, and it is capable of easy and foolproof operation even in the hands of an inexperienced workman. Y

Further advantages and features of the process and apparatus of the invention will be evident from the following description, when the description is considered in conjunction with the accompanying drawings in which:

Figure l is a schematic representation of apparatus constituting an embodiment of the present invention, and this representation shows mostly in block form various components which are suitable for carrying out the irnproved process of the invention;

Figure 2 is a circuit diagram of appropriate circuitry for the components shown in block form in Figure l; and

Figures 3 and 4 are series of curves showing the various electrical signals and pulses that are produced in the apparatus of Figures 1 and 2, and which are used for carrying out the process of the invention.

The apparatus of Figure l includes an anodizing'bath 10 of usual construction. This bath, for example, may be lined with a lead lining l1, and the lead lining constitutes in known manner the cathode for the anodizing process. A series of refrigeration cooling coils 12 are wound about the interior of the bath l@ adjacent the lining l1. These coils have a refrigerant passing through them from any suitable refrigerating apparatus. Most commercially available refrigerators are suitable for the purpose, and it is believed unnecessary to show or describe the details of a typical refrigeration unit. The use of refrigeration coils in the electrolyte of an anodizing bath per se is known.

The bath it) is intended to hold a suitable electrolyte for anodizing purposes in electrical contact with the lead lining 1l. The workpiece to be anodized is indicated as 14, and this vworkpiece is suspended by any suitable bracket means (not shown) in the electrolyte contained in the bath l0. Although anodizing is usually carried out on aluminum bodies and bodies composed of aluminum alloys, other materials also are susceptible to anodizing. For example, as noted above, bodies composed of magnesium and zinc, and other materials, have been successfully anodized.

The electrical components of the apparatus shown in Figure 1 include a source of alternating current 15. This source may be the usual domestic 11S-volt, 60-cycle alternating current mains. The voltage from the source l5 is introduced to a phase-shifting circuit l5, and this circuit is coupled to a pulsing circuit 18. The pulsing circuit 18 is connected between the control grid and cathode of a gaseous discharge tube 2t?.

The discharge tube 20 may be of the type presentiy referred to as a thyratron, or an ignitron, although any other suitable known type of discharge tube can be used. The anode of the discharge tube 29 is connected to one side of the secondary winding of a transformer 2.7L,l and the other side of this winding is connected to the lining 11 of the anodizing bath 10. The cathode of the tube 29 is connected to the workpiece i4 to be anodized over the indicated ground connection. A pair of output terminals 25 are included in these connections. As mentioned above, the workpiece 14 forms the anode in the electrolyte and the metal lining 11 of the bath 10 forms the cathode. The primary winding of the'transformer 22 is connected to a source of laternating current 24 which may have the same characteristics as the source 15.

In a manner to be described lin detail subsequently, the pulsing circuit 18 produces triggering pulses for the gas discharge tube 20. The phase shifting circuit 16 is manually controllable, so that the discharge .tube 20 may be triggered at any point on the positive cycle of its anode voltage, as supplied from the source 24. By the control ofthe triggering of the gas tube 20, discrete uni-directional current pulses are established through the secondary of the transformer 22, and these pulses flow to the anode 14 through the electrolyte in the bath 10 from the cathode formed by the lining 11 of the tank enclosing that bath. Of course, a separate cathode immersed in the bath could be used. The resulting current pulses flowing through the electrolyte can be controlled as to amplitude and as to their individual duration Vby the control of the phase shifting circuit 16. The most satisfactory operation is obtained when the current pulses have arelatively large amplitude for high current density, and when there is a relatively long interval between each successive pulse for adequate cooling.

In a particular application of the process of the invention, the following conditions were present:

-Workpiece 14 Aluminum.

Electrolyte Sulphuric acid 11% by weight. Oxalic acid 1% by weight.

Electrolyte temperature 38-42 Fahrenheit.

Current density 200 amps. per sq. foot.

Anodizing time l0 minutes.

Film thickness .C02-.0035 inch.

Current pulse duration (percentage of complete cycle).

' connected to the grounded end of the resistor 60 and to the center tap of the secondary winding of the transformer 50.

The anode of the diode 58 is connected to the control grid of a pentode 64. Thercathode of the pentode is grounded, and its anode is connected to one terminal of a resistor 66. The vsuppressor electrode of the pentode 64 is connected to its cathode, and the screen electrode is connected to one terminal of a resistor 68. A grounded by-passing capacitor 70 is connected to the screen electrode. The other terminal of the resistor 66 and of the resistor 68 are both connected to the movable tap of a potentiometer 72. This potentiometer is connected inV the output circuit of a usual power supply which will be described and which supplies positive unidirectional exciting potentials to the pentode 64.

A capacitor 74 is coupled between the anode of the pentode'64 and the control grid of the gas discharge tube 20 referred to previously. As noted above, the

anode of the tube 20 is connected to one side of the secondary winding of the transformer 22, and the cathode of the tube is grounded. The output terminals 25, as described above, are connected to the other side of the secondary winding of thertransformer 22 and to ground. These terminals, as described, are connected to the anode 14 and to the lining 11 of the bath 10.

The power supply includes a usual power transformer 76 whose primary winding is connected to the alternating current source. The secondary winding 77 of the transformer 76 is connected to the anodes of a fullwave rectifier tube 78, and the center tap of the secondary winding is grounded. The transformer 76 also includes a filamentV winding 79, and the filament winding is connected to the filament of the rectifier tube 78. The center tap of the filament winding 79 is connected to one side of a choke coil 80, and the other side of the choke coil 80 is connected to one of the fixed contacts of the potentiometer 72. The other fixed contact of the potentiometer is grounded. A lt'er capacitor 82 is shunted across the potentiometer 72.

In a usual manner, the rectifier 78 rectifies the voltage appearing across the secondary of the power transformer 76. The rectifier voltage is filtered by the elements 80 and 32, and a positive directl voltage appears across the potentiometer 72. Then,V by adjustment vof the movable tap on `the potentiometer 72,:the proper positive directv exciting voltage can be supplied to the anode and screen grid of the pentode 64.

A half-wave rectifier tube 84 is connected to one side of the secondary 77 of the power transformer 76. This latter rectifier may bel a simple diode whose cathode is connected to the secondary winding and whose anode is connected to a choke coil 86. The choke coil, in turn, is connected to one of the fixed contacts of a potentiometer 88. The other fixed contact of the potentiometer 88 is grounded, and the potentiometer is shunted by a filter capacitor 90. The movable 'tap of the potentiometer 88 is connected to a resistor 92 which, in turn, is connected to the control grid of the'gas discharge tube 20.

ln known manner, a negative direct voltage is developed across the potentiometer 88, and this voltage is used to provide the appropriate adjustable bias for the gas discharge tube 20. This negative bias is controllable merely by shifting the movabletap on the potentiometer'88.

The voltage appearing across the secondary of the transformer 22 is shown, for example, in curve A in Figure 3. This voltage vmay be a usual 60-cycle alternating-current sinusoidal wave. The voltage across the secondary is introduced to they anode of the gas discharge tube 20. As is `well known, the discharge tube 20 can be fired only on the positive half-cycles of this voltage wave, when the anode is driven positive with respect to the grounded cathode.

The pentode 64 is normally conductive so that a relatively low voltage appears at its anode. The-phase shifting circuit 16 operates in known manner to produce a pair of signals, which are mixed across the resistor 60. One of the signals is phase shifted by the capacitor 54, and the other has an amplitude which is controllable by adjustment of the resistor 52. The result is a peaked complex wave across the resistor 60 whose peak may be shifted with respectV to a time base merely by shifting the value of the resistor 52. The negative half-cycle only of the complex wave across the resistor 60 is introduced to the control grid of the pentode 64 due to the action of the diode 58. In this manner, the pentode 64 may be periodically rendered non-conductive' and at a point in each cycle of the complex wave, as determined by the adjustment of the resistor 52.

Each time the pentode 64 is rendered non-conductive,

it introduces a positive'. triggering pulse to the control' grid of the gas discharge tube 20. These triggering pulses are shown in the curve B of Figure 3. The peaks of the' pulses of the curve B may be made to occur at i any time during respective ones of the positive half-cycles of the curve A in Figure 3 under the control of the resistor 52, as noted above.

When each of the pulses of the curve B fires the gas discharge tube 20, the discharge of the tube continues during each triggering cycle until the corresponding cycle of the voltage wave of curve A introduced to its anode swings negative. This is in accordance with the well known triggering of thyratrons or other gas discharge tubes. The resulting plate voltage on the gas discharge tube Z is shown in curve C of Figure 3', and the current pulses passed through the bath of Figure 1 are shown in the curve D. Merely by controlling the point of each positive half-cycle of the curve A at which the triggering pulses of the curve B trigger the gas discharge tube 2t?, the individual duration and amplitude of the pulses of curve D can be controlled. This is effectuated under the co-ntrol of the resistor 52, as described above. This enables the characteristics of the current pulses passed through the bath 10 to be tailored to meet the needs of the material being processed at any particular time.

The effect on the currentpulses through the bath as the control 52 is varied is' shown in Figure 4. The curve A of Figure 4 shows the in phase condition whereby the current pulse is 100% of thepositive half cycle of the voltage across the secondary of the transformer 22. The curve B of Figure 4 showsa condition similar to that of the curve D of Figure 3, in which the control 52 is varied so that current flows through the bath for about 50% of the positive half cycle. Finally, the curve C of Figure 4 shows the condition of the control 52 in which the current pulses flow through the bath at reduced amplitude and for a small .percentage of the half cycles.

The curves of Figure 4 demonstrate the extreme flexibility of theV system, and how the amplitude of the individual current pulses can be simply controlled, as well as the cooling intervals between successive ones of the pulses. Typical operation is represented by the condition of the curve B of Figure 4 in which current liows through the bath 1t) Vfor about 25% of the full cycle of voltage across the secondary of the transformer 22. This control may be carried out manually or automatically for the diiierent substances to be anodized. Moreover, the co-ntrol can be varied for optimum anodizing results during the anodizing process of any particular substance. An indication by a cathode-ray oscilloscope, or similar instrument, of the wave shape of the current pulses may serve as a convenient monitor of the desired condition of the anodizingV current at any particular time.

The gaseous discharge tube may have one or more tubes connected in parallel with it to increase the current capabilities of the system. Also, by connecting a pair of tubes such as the tube 20 in a known back-toback circuit, and by the use of a tapped output transformer, negative current pulses can also be produced through the bath 14D in addition to the positive current pulses discussed above. These negative current pulses may be controlled to have a reduced amplitude, and they may serve further to reduce the effects of anode polarization.

The process of the invention oifers a wider range of metallic alloys that can be anodized successfully. The process greatly reduces the possibility of burning the workpiece. in addition to this, the process increases the rate at which the anodic lrn is built up, as compared to the prior art processes.

The present invention provides, therefore, an improved anodizing process and apparatus in which the processing speed of the workpiece being anodized may be materially increased. This results in a harder anodic lilm because it is subject to the action of the electrolyte for ashorter length of time, as compared with-the prior art processes.

Also, the apparatus and process of the inventionY provides for a cooling interval atthe anode workpiece to permit an increase in current density further to enhance 8 the hardness characteristics of the anodic iilm formed at the anode and to increase the obtainable thickness of the film.

The refrigeration rf'zquiremeritsl of the system are materially reduced as compared to the prior art processes, this being due to the anode cooling by the pulsing action ofthe apparatus and process of'the invention.

The actual electrical control system and components of the apparatus of the invention are relatively simple in their construction, and the individual circuits are in themselves simple to construct and they use well known component parts. The system is readily adjustable to lit the needs of any particular material or alloy being processed, and it is straight-forward and foolproof in its operation.

Also, the improved s'ytem and process of the invention is extremely economical and eiiicient in power consumption. A constructedV embodiment of the invention, for example, was found to require essentially lifty percent of the amount of power required by a typical prior art system.

I claim:

l. An anodizing method for forming a relatively thick and hard inorganic corrosion-resistant non-reactive anodic iilm on an electrically conductive body capable of being anodized which includes: providing an electrolyte having a tendency to dissolve the anodic oxide film on the electrically conductive body, the rate of which dissolution increases with increase in the temperature of the electrolyte at the surface of the body, immersing at least a portion of the electrically conductivebody in the electrolyte, and passingl a series of uni-directional electrical current pulses of a repetition frequency of about pulses per second through the electrolyte and throughthe electrically conductive body to provide an anodic oxide film on the electrically conductive body, with each successive pulse penetrating the oxide film to the electrically conductive body and with each of said unidirectional current pulses being separated by an interval of essentially zero current flow through the electrolyte and through the electrically conductive'body, with the interval between each of such current pulses beirigof a duration approximately three times the duration of the interval of each of the current pulses to provide a cooling period at the surface of the immersed portion of the electrically conductive body between each of said current pulses, the cooling period between each current pulse being approximately three times the duration of each current pulse.

2. The method defined in claim l which further includes refrigerating the electrolyte to a temperature less than 45 degrees Fahrenheit.

3. An anodizing method of forming a relatively thick and hard inorganic corrosion-resistant non-reactive anodic film on an electrically conductive body capable of being anodized which includes: providing an electrolytehaving a tendency to dissolve the anodic oxide film on the electrically conductive body, the rate of which dissolution increases with increase in the temperature of the electrolyte at the surface of the body, immersing at least a portion of the electrically conductive body in the electrolyte, and passing a series of uni-directional electrical current pulses of a selected repetition frequency through the electrolyte and through the electrically conductive body to provide an anodic oxide film on the electrically conductive body, with each successive pulse penetrating the oxide iilm to the electrically conductive body and with each of said pulses being separated by an interval of essentially zero current ow through the electrolyte and through the electrically conductive body, with the duration of each of such current pulses constituting up to approximately 25 percent of the total repetition period of such pulses to provide an interval between successive ones of the current pulses so as to establish a cooling period at the surface of the immersed portion of the electrically conductive body between cach of said current pulses.

4. An anodizing method for forming a relatively thick and hard inorganic corrosion-resistant non-reactive anodic 'ilm on an electrically conductive body capable of being anodized which includes: providing an electrolyte having a tendency to dissolve the anodic oxide lilm on the electrically conductive body, the rate of which dissolution increases with increase in the temperature of the electrolyte at the surface of the body, immersing at least a portion of the electrically conductive body in the electrolyte, and passing a series of uni-directional electrical current pulses of a selected repetition frequency through the electrolyte and through the electrically conductive body to provide an anodic oxide iilm on the electrically conductive body, with each successive pulse penetrating the oxide film to the electrically conductive body and with each of said pulses being separated by an interval of essentially zero current ow through the electrolyte and through the electrically conductive body, with the duration of each of such current pulses constituting approximately 25 percent of the total repetition period of such pulses to provide an interval between each such current pulses of a duration approximately three times the duration of the interval of each of the current pulses so as to establish a cooling period at the surface of the immersed portion of the electrically conductive body between each of said current pulses which is approximately three times the duration of each current pulse.

5. An anodizing method for forming a relatively thick and hard inorganic corrosion-resistant non-reactive anodic film on an electrically conductive body capable of being anodized which includes: providing an electrolyte having a tendency to dissolve the anodic oxide film on the electrically conductive body, the rate of which dissolution increases with increase in the temperature of the electrolyte at the surface of the body, immersing at least a portion of the electrically conductive body in the electrolyte, and passing a series of uni-directional electrical current pulses of a repetition frequency of about pulses per second through the electrolyte and through the electrically conductive body to provide an anodic oxide iilm on the electrically conductive body, with each successive pulse penerating the oxide lm to the electrically conductive body and with each of said unidirectional current pulses being separated by an interval of essentially zero current flow through the electrolyte and through the electrically conductive body, with the duration of each of the current pulses constituting up to approximately 25 percent of the total repetition period of such pulses to provide an interval between each of such current pulses of a duration at least three times the duration of the interval of each of the current pulses so as to establish a cooling period at the surface of the immersed portion of the electrically conductive body between each of said current pulses which is at least twice the duration of each current pulse.

References Cited in the le of this patent UNITED STATES PATENTS 2,396,685 Coggins Mar. 19, 1946 2,541,275 Odier Feb. 13, 1951 2,582,020 Emery Jan. 8, 1952 2,692,851 Burrows Oct. 26, 1954 2,715,095 Cohn Aug. 9, 1955 2,726,203 Rockafellow Dec. 6, 1955 FOREIGN PATENTS 636,665 Germany Oct. 13, 1936 l716,554 Great Britain Oct. 6, 1954` 

1. AN ANODIZING METHOD FOR FORMING A RELATIVELY THICK AND HARD INORGANIC CORROSION RESISTANT NON-REACTIVE ANODIC FILM ON AN ELECTRICALLY CONDUCTIVE BODY CAPABLE OF BEING ANODIZED WHICH INCLUDES: PROVIDING AN ELECTROLYTE HAVING A TENDENCY TO DISSOLVE THE ANODIC OXIDE FILM ON THE ELECTRICALLY CONDUCTIVE BODY, THE RATE OF WHICH DISSOLUTION INCREASES WITH INCREASE IN THE TEMPERATURE OF THE ELECTROLYTE AT THE SURFACE OF THE BODY, IMMERSING AT LEAST A PORTION OF THE ELECTRICALLY CONDUCTIVE BODY IN THE ELECTROLYTE, AND PASSING A SERIES OF UNI-DIRECTIONAL ELECTRICAL CURRENT PULSES OF A REPETITION FREQUENCY OF ABOUT 60 PULSES PER SECOND THROUGH THE ELECTROLYTE AND THROUGH THE ELECTRICALLY CONDUCTIVE BODY TO PROVIDE AN ANODIC OXIDE FILM ON THE ELECTRICALLY CONDUCTIVE BODY, WITH EACH SURCESSIVE PULSE PENETRATING THE OXIDE FILM TO THE ELECTRICALLY CONDUCTIVE BODY AND WITH EACH OF SAID UNIDIRECTIONAL CURRENT PULSES BEING SEPARATED BY AN INTERVAL OF ESSENTAILLY ZERO CURRENT FLOW THROUGH THE ELECTROLYTE AND THROUGH THE ELECTRICALLY CONDUCTIVE BODY, WITH THE INTERVAL BETWEEN EACH OF SUCH CURRENT PULSES BEING OF A DURATION APPROXIMATELY THREE TIMES THE DURATION OF THE INTERVAL OF EACH OF THE CURRENT PULSES TO PROVIDE A COOLING PERIOD AT THE SURFACE OF THE IMMERSED PORTION OF THE ELECTRICALLY CONDUCTIVE BODY BETWEEN EACH OF SAID CURRENT PULSES, THE COOLING PERIOD BETWEEN EACH CURRENT PULSES BEING APPROXIMATELY THREE TIMES THE DURATION OF EACH CURRENT PULSE. 